Ferroelectric Engineered Electrode‐Composite Polymer Electrolyte Interfaces for All‐Solid‐State Sodium Metal Battery

Abstract To enhance the compatibility between the polymer‐based electrolytes and electrodes, and promote the interfacial ion conduction, a novel approach to engineer the interfaces between all‐solid‐state composite polymer electrolyte and electrodes using thin layers of ferroelectrics is introduced. The well‐designed and ferroelectric‐engineered composite polymer electrolyte demonstrates an attractive ionic conductivity of 7.9 × 10–5 S cm–1 at room temperature. Furthermore, the ferroelectric engineering is able to effectively suppress the growth of solid electrolyte interphase (SEI) at the interface between polymer electrolytes and Na metal electrodes, and it can also enhance the ion diffusion across the electrolyte‐ferroelectric‐cathode/anode interfaces. Notably, an extraordinarily high discharge capacity of 160.3 mAh g–1, with 97.4% in retention, is achieved in the ferroelectric‐engineered all‐solid‐state Na metal cell after 165 cycles at room temperature. Moreover, outstanding stability is demonstrated that a high discharge capacity retention of 86.0% is achieved over 180 full charge/discharge cycles, even though the cell has been aged for 2 months. This work provides new insights in enhancing the long‐cyclability and stability of solid‐state rechargeable batteries.

. Cross-sectional (a) morphology, (b) Nb and (c) Si mapping of KNN-NZSP ceramic framework.  Figure S3. Current-potential curves of (a) non-engineered NZSP CPE, and (b~d) ferroelectric-engineered KNN-NZSP CPE-1L, 2L and 3L, measured at room temperature with various sweeping rates in the potential sequence from open circuit potential (OCP) to -2.0 V and back from -2.0 V to 6.0 V (vs. Na/Na + ). Figure S4. Electrochemical impedance spectroscopies of the NVP | CPE | Na cells with the NZSP CPE and the KNN-NZSP CPE-1L, 2L and 3L, (a) before and (b) after long-term cycling. Figure S5. (a) Coulombic efficiency of the NVP | electrolyte | Na cells with LE, NZSP CPE and KNN-NZSP CPE in long-term cycling; (b) 1 st galvanostatic charge-discharge profile of the NVP | KNN-NZSP CPE | Na cell. Considering the narrow electrochemical window of LE, the test voltage range is 0.8~4.2 V for the LE cell, and 0.8~4.5 V for the NZSP CPE cell and the KNN-NZSP CPE cell. All measurements were conducted at room temperature. Figure S6. Charge-discharge profiles of the (a) NVP | NZSP CPE | Na cell and (b) NVP | KNN-NZSP CPE | Na cell before and after long-term suspension. All measurements were conducted at room temperature.
One dimensional finite element model is set up to investigate the influence of KNN engineering at the electrolyte-electrode interfaces. In the simulation model, only the electrodeelectrolyte interfaces are studied, and the thicknesses of the interfacial cathode, anode and electrolyte are all set as 5 , while the thickness of KNN as 0.7 . Outside the interfaces, to simplify the model, the electric potential and ion concentration are all treated as constant.
The current density J within the model can be expressed as: where c is the molar ion concentration, D is effective diffusivity, F is Faraday constant, R is gas constant, T is temperature and E is the built-in electric filed generated by KNN.
By solving the above partial differential equations, the ion concentration distribution can be obtained, and then the electric potential can be calculated by ion concentration.
Here, the KNN-induced electric field, E, mainly depends on its polarizations. We treat the oriented KNN single domain as a pair of point charges, and one end with charge +q and the other one with charge -q. The displacement vector ⃗ can be regarded as negative to positive.
Then the dipole moment can be written as: Assuming the location of +q is + ⃗⃗⃗⃗ while the location of -q is − ⃗⃗⃗⃗, then ⃗ = + ⃗⃗⃗⃗ − − ⃗⃗⃗⃗.
The electric potential at location ⃗ can be expressed as: Let ⃗⃗ as the position vector relative to the mid-point + ⃗⃗⃗⃗⃗+ − ⃗⃗⃗⃗⃗ 2 , and ⃗ is the corresponding unit vector: By Taylor Expansion and omitting the terms of higher order, the potential can be expressed as: Then the electric field is the negative gradient of the potential, leading to The total electric field of KNN coating can be expressed as: After charging under 3.5 V and 3.7 V, the NVP | NZSP CPE | Na cell is further charged under 4.0 V to reach its full-charged state. Figure S8 shows the impedance changes of the cell under 4.0 V long-term charging. As a comparison, the NVP | KNN-NZSP CPE | Na cell is also charged under 4.0 V. However, to avoid over-charging, the KNN-NZSP CPE cell has been fully discharged before the 4.0 V-charging. The impedance plots of the NVP | CPE with/without KNN | Na cells in Figure 6 and Figure S8 are well fitted with the inserted equivalent circuits.
In the equivalent circuit, R1 represents the electronic resistance of the cell. (R2 || C1) correspond to the electrolyte resistance (the semicircle at high frequency, 10 6~1 0 5 Hz) and the estimated C1 is ~10 -9 F. R3 and the parallel CPE1 (~10 -6 F) correspond to the SEI resistance and its fitted semicircle is at low frequency with characteristic frequency of 10~100 Hz, whereas R4 || C2 (the semicircle at middle frequency whose characteristic frequency is 10 4~1 0 5 Hz) represents the charge transfer resistance at anode-electrolyte interface. [S1] The fitted (R3 || CPE1) and (R4 || C2) values agree well with the fitting results in the symmetric Na | CPE | Na cells ( Figure S9). On the other hand, the elements (R5 || C3) and (R6 || C4) represent the CEI and cathode-electrolyte charge transfer resistances, respectively. The characteristic frequency of the fitted semicircle is ~10 2 Hz for the cathode-electrolyte charge transfer, and is ~10 3 Hz for the CEI component. [S2, S3] As shown in Figure S8  In Figure S9, when Na | CPE | Na cells are under high-voltage charging, Na + migrates from positive Na (cathode) to negative Na (anode), and would combine with the accumulated electrons at the interfaces to transform into Na. The newly formed Na would react with the electrolyte, producing the oxide SEI/CEI at the anode/cathode-electrolyte interfaces. Thus, huge SEI and CEI resistances are demonstrated in the Na | NZSP CPE | Na cell, which empirically agrees with that in the NVP | CPE | Na cells, considering the individual specificity of the cells. However, the ferroelectric KNN presents its good capability in controlling the interfacial space charges, and it could also enhance the compatibility between CPEs and Na metal electrode. Therefore, greatly reduced interfacial resistances are achieved in the Na | KNN-NZSP CPE | Na cell.